1. Field of the Invention
The invention relates to controls for multiple-ratio, geared transmissions, particularly for automotive vehicles, to effect a smooth transition between ratios during a shift sequence.
2. Background Art
A conventional multiple-ratio automatic transmission for automotive vehicles typically includes a hydrokinetic torque converter located between the crankshaft of an internal combustion engine and the multiple-ratio gearing. The gearing establishes multiple torque flow paths to the vehicle traction wheels. The ratio changes that occur may be accomplished by simultaneously engaging a drive clutch and disengaging a reaction brake, the reaction brake establishing a reaction point for the gearing. Such a shift is referred to as a synchronous shift. A non-synchronous shift may include an overrunning coupling for establishing a reaction point for the gearing during torque transfer and a separate coupling for clutching together two elements of the planetary gearing. An example of a transmission of this kind may be seen by referring to U.S. Pat. No. 6,122,583, which is assigned to the assignee of the present invention.
It is known also in the automotive transmission art to provide geared multiple-ratio transmissions in a torque delivery driveline wherein a xe2x80x9cswap shiftxe2x80x9d ratio change sequence can be achieved as a reaction brake for one reaction gear is applied and an independent brake for a second gear is released. An example of a transmission having a xe2x80x9cswap shiftxe2x80x9d feature may be seen by referring to U.S. Pat. No. 6,301,538. This patent also is assigned to the assignee of the present invention.
Ratio changes are controlled by an electronic microprocessor that develops control signals in response to changes in operating variables of the powertrain to actuate shift solenoids, which, in turn, control shift valves for actuating and releasing clutch and brake actuators.
The present invention comprises a pressure control system and control method for an automatic transmission wherein control elements for the transmission clutches and brakes are controlled independently by controlling pressure profiles for each clutch or brake element. A seamless transition between ratios is provided by pre-staging shifts during a ratio change sequence.
The system architecture, which is in modular form, incorporates several control features of conventional transmissions. It is configured, however, to achieve a shift sequence in which the destination gear is obtained as quickly as possible, both during a regular ratio change sequence and during a so-called change-of-mind sequence where a new destination gear is commanded before the completion of a previously commanded destination gear. The overall system response is improved because of this pre-staging capability of looking ahead for pending shifts. This permits the controller to prepare in advance the clutch or brake actuators to be used during the shift sequence.
During a shift sequence, the current shift is performed during a so-called stage 1 shift. A pending shift, which is referred to in this disclosure as a stage 2 shift, is prepared during the progress of the stage 1 shift. The stage 2 shift is transformed into stage 1 when the current stage 1 shift is completed. Any new shift in the shift sequence needed to achieve the final destination gear is loaded into stage 2. At this point, shifts will continue to be transposed or rolled over from stage 2 to stage 1 as they are completed until the final destination gear is achieved.
The pre-staging of the ratio changes during a shift sequence reduces delays caused by the hydraulics and the electronics of the control system. The controller includes a modular library of shifting and pre-staging pressure profiles. On startup, the strategy detects the current gear ratio to initialize the system into a proper gear after the transmission is powered up.
A change-of-mind shift, in which the destination gear is in the opposite direction from the current shift, causes the current shift to be interrupted as the system returns to the starting gear while pre-staging elements needed for the final desired gear. For a change-of-mind situation in which a shift to a destination gear is in the same direction as the current shift, the strategy will continue with the current shift while pre-staging elements needed for a final desired gear.
The invention includes a first system and method step for processing changes in the desired destination gear ratio to identify active friction elements and time the start and completion of each ratio change.
A second subsystem and method step specifies the pressure control action required to apply or release a friction element during a ratio change and during a friction element engagement. The second subsystem has a library of pressure profiles required to complete all ratio changes and engagements.
A third subsystem and method step involves storing a collection of pressure calculations and algorithms accessible by the first and second subsystems and method steps for calculating an appropriate pressure for each friction element.
The first and second subsystems interface with the third subsystem and with each other. The first and second subsystems use calculations in the third subsystem to monitor the status of a ratio change and to calculate the appropriate output pressure for each friction element.
Each subsystem may be a separate module that is independently configured whereby the control system is adaptable by calibration for use with a variety of transmission mechanisms with discrete pressure actuated friction elements and for specifying pressure profiles, friction element actuation and gear ratio sequencing.
The first subsystem has buffer portions that receive desired friction element data for a current gear in a first buffer stage of a gear shift sequence, and buffer portions that receive desired friction element data for a forthcoming gear in a second buffer stage of a gear shift sequence.